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Solid State Relay - Need help!

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Hi ensemble,


I need some help with an issue related to Solid State Relay.


The machine is composed of:


- a CNC router/hot wire cutting combo

- a chopper board to control stepper motors and Z level probe (parallel port #1)

- a supply for wire heating

- a breakout board to control two Solid State Relays (parallel port #2, USB for 5V SSR supply)

- a Solid State Relay to control the wire heating (DC-DC)

- a Solid State Relay to control spindle on/off (DC-AC, 10A)

- a spindle (Kress, 4.8A)

- control wires are shielded and grounded


The machine works well and has several hours of work on its shoulders, both on milling or hot wire cutting.

Both control board seems to work properly.

The DC-DC SSR works well controlling wire heating

The DC-AC works well controlling a vacuum cleaner, even if I set the cleaner at partial power.

The spindle works well if I connect it directly to mains supply, at partial speed also.


The problem arise when I try to control the spindle with the SSR.

If I set the spindle at full speed, with the on board knob, it works well.

If I set the spindle at partial speed, revolution speed jumps a lot. Definitely not usable and even dangerous. The bit could break, ruining the piece or worst...

If I control the SSR with a LIPO 3S battery, there are no issues.

Voltage measured at control line with a multimeter is stable at around 4.2V.

(I should find the divider I built several years ago and check the control line feed with a PC sound card oscilloscope)


At this point I'm not sure if it is the spindle that creates so much electrical noise, to disturb so much the breakout board, or the line, or the SSR.

I tried adding capacitors on the control wire, SSR side, and on USB 5V line on breakout board side, with no noticeable improvement.

I tried feeding USB from a wall charger and even another mains supply, no changes.


I'm not sure which solution could be the better.

Using a FET, as suggested by Tilman (thanks!)?

Going for another SSR, maybe a phase controlled one?

Changing the breakout board and using a better shielded one?

Last one continuing with manual control ... (nooooo...)


In the case of the FET solution, should I create a filtered power feed, or just connect it to a suited power supply? A schematics would be helpful.


I'm becoming crazy with that and developments suffer from this waste of time :angry:


Any help will be really appreciated: thanks in advance!

Best regards,


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A: I wish I had one of those.


B: I wish I knew what you were talking about so that I could help.


C: Maybe if I had one of those, I could do some 'fault-finding' and help you out!


D: I also wish I could create digital aircraft that could be produced (or their moulds) on that piece of kit.


E: there are too many facets to this hobby.


I'm sure you have sorted out by now.

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Tilman Baumann

Lol could not agree more to point E.

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I'm sure you have sorted out by now.

Unfortunately not :blush:

The last cuts for pieces to complete three more Foka4 were done just with manual on/off of the spindle.

As this is not prioritary I'm taking it easy.


Thanks for the nice comment! Really appreciated :)


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